This invention relates to an improved construction of a housing and parts attached thereto to form a test piece gripping device, and more particularly to improvements in gripping devices of the type used in conjunction with physical testing apparatus for gripping the opposite ends of a test specimen undergoing a pull test.
As is known, gripping devices of the type described are used in conjunction with physical testing apparatus wherein two of the gripping devices are used to grip the opposite ends of a test specimen. The gripping devices are forced apart by the power components of the physical testing apparatus to subject the test specimen to various tensile forces. Among the physical properties of the test specimen, which are to be determined in this manner, are yield strength, ultimate strength and breaking stress. As is known in the art and specifically, for example, as shown in my prior U.S. Pat. No. 3,403,549, such gripping devices each consists of a housing having oppositely-inclined side walls. A jaw member is engaged with and slideable over each of the inclined walls. The inclination of the side walls is such that the jaw members are moved in one direction as they are spread apart so as to receive the end of a specimen, and as the jaw members are moved in the opposite direction, they approach each other to firmly grip the test specimen. Spring means act on the jaw members to urge them into the grip position. While the gripping device disclosed in my prior patent has many desirable features and advantages, it nevertheless suffers from certain disadvantages which the present invention is designed to overcome. One of these disadvantages is the need to execute a number of intricate and extensive machining operations to produce the housing used to operatively support various parts forming the gripping device and sustain the forces developed between the gripper jaws. The housing disclosed has a keystone configuration with outer side surfaces converging toward a lower end surface where the ends of the gripper jaws are exposed to receive a test piece between the jaws after they are separated. A deep hollowed-out portion in the front face of the housing essentially conforms to the keystone configuration of the housing itself so as to form inclined side walls for sliding engagement with the jaw members. The jaws each includes an inclined slot to receive a single cam member that is held in place by a cover plate. The cover plate is secured to a ledge positioned downwardly from the face surface of the housing. A deep chamfered edge was formed between the outer face surface of the housing and the face surface of the cover plate. The cover plate further includes two centrally-arranged slots, one extending to the upper end of the cover plate and the other extending to the lower end of the cover plate. The upper slot formed a guide for a pin extending from the cam member while the lower slot exposed the jaw members for observation during the clamping operation. The front and back face surfaces of the housing were each tapered into two directions by a laborious machine operation whereby the thickness of the housing increased from its top surface to a maximum thickness at about the top of the cavity in the housing and then decreased to a minimum thickness at the lower edge of the housing. The many maching operations required to produce such keystone-shaped housing with tapered front and back surfaces, as described, contribute materially to the cost of the gripping device. Moreover, the arrangement of parts limited the observation of the jaws during the gripping operation and rendered unnecessarily complicated the procedure required for replacing the gripper jaws.
Substantial amounts of metal must be removed from the housing in order to provide the required inclined side walls to support the jaw members for movement into and out of engagement with the end of a test piece. The amount of material removed from the housing, of course, reduces the maximum stress to which the gripping device may be subjected without breaking. The stresses are applied to the housing by a pull shaft. This shaft is secured by a threaded connection forming a first critical area in the housing which is the end opposite the end where the gripper jaws are exposed for receiving the test piece. There is, of course, a pull axis extending symmetrically through the gripping device. The housing to which the pull shaft is secured must be capable of withstanding the maximum stresses occurring during the actual testing procedure. At the same time, the housing is subjected to a stress produced by the separating force between the jaws. This represents a second critical area where the housing must resist maximum stresses.